Alloy steel and its preparation



United States Patent Oilice 3,365,342 ALLOY STEEL AND ITS PREPARATEON Lee S. Richardson, West Chester, and William R. Patterson, Mendenhall, Pa, assignors to Foote Mineral Company, Exton, Pa., a corporation of Pennsylvania No Drawing. Filed June 15, 1965, Ser. No. 464,221 Claims. (Cl. '75123) The present invention relates to a novel alloy steel possessing very high strength coupled with toughness and ductility and to its preparation.

There is a class of ferrous-base alloys containing nickel with interrelated amounts of cobalt and molybdenum which can be age-hardened while in the martensitic condition. These alloys are known as maraging steels and are characterized by very high strength coupled with high toughness and ductility. A common commercial variety of these alloys is the so-called 200 grade 18% nickel maraging steel which contains 17-19% nickel, 89% cobalt, 3-3.5% molybdenum, 0.15-0.25% titanium, about 0.1% aluminum, less than 0.03% carbon, less than 0.1% manganese, less than 0.1% silicon and the balance essentially iron. This steel has an ultimate tensile strength of about 200,000 p.s.i.; a high toughness, as shown by the ratio of notch tensile strength to ultimate tensile strength of about 1.5; good ductility, as shown by an elongation (in 1 inch using a 4" diameter tensile test bar) of about 11-15%, and a reduction in area of 55-65%.

United States Patent 3,132,938 appears to be directed to the foregoing alloy.

In copending application Ser. No. 461,517, filed June 4, 1965, are disclosed and claimed a maraging alloy steel having a strength, toughness and ductility comparable to that of 18% nickel maraging steel but consisting essentially of, in percent by weight, from 11 to 14% nickel, from 6 to 9% cobalt, from 3 to 5% of molybdenum, from 1.5 to 3% of manganese, no more than 0.2% aluminum, no more than 0.4% titanium, carbon in an amount less than 0.03%, less than 0.15 silicon and the balance essentially iron. In an embodiment where manganese is present in an amount between 2.5 and 3%, vanadium is also included in an amount up to 0.4%.

Since iron is the least expensive constituent of alloy steels it is desirable to maintain the amount of alloy constituents, especially the more expensive ones, at the lowest possible concentration consistent with desired properties.

It is the principal object of the present invention to provide a novel strong, tough and ductile nickeland manganese-containing ferrous base alloy of the maraging type.

It is another object of the present invention to provide a novel maraging steel alloy having a strength, toughness and ductility approaching or comparable to conventional 200 grade 18% nickel maraging steel but less expensive to manufacture by virtue of a substantially lower content of alloy constituents.

It is a further object of the present invention to provide a method for manufacturing the novel alloy steel.

Other objects will become apparent from a consideration of the following specification and claims.

The novel age-hardened, martensitic alloy steel of the present invention consists essentially of, in percent by weight, from 6 to 13% of nickel, from 2 to 6% of molybdenum, from 1.5 to 4% of manganese, no more than 0.5% of vanadium, no more than 0.3% of aluminum, no more than 0.4% of titanium, carbon in an amount less than 0.03 less than 0.15% of silicon and the balance essentially iron. In the present alloys, the manganese equivalent (the manganese content plus one-third the nickel content) should be at least 5.5, preferably from 5.5 to 6.2.

The foregoing age-hardened, martensitic alloy steels 3,365,342 ?atentecl Jan. 23, 1968 possess high strength coupled with toughness and ductility, and the preferred alloy steels possess strength, toughness and ductility approaching or comparable to those of presently available 200 grade 18% nickel maraging steel. Yet, because it contains much less alloying constituents the present alloy steel can be prepared at less cost than the conventional 200 grade 18% nickel maraging steel. This alloy contains an optimum combination of nickel and manganese which can promote the transformation to a martensitic but age-hardenable structure using a minimum of alloying constituents. This alloy also contains an optimum combination of molybdenum and other alloying constituents to induce age-hardening thus providing the desired mechanical properties.

The present alloy may be prepared in the same manner as conventional 18% nickel maraging steel using, however, constituents and amounts thereof as specified herein. Advantageously, at least the principal source of manganese, and preferably the entire source, will be electrolytic manganese. Likewise in selecting sources of the other constituents care will be observed in selecting those sufliciently low in carbon and also in silicon. Electrolytic iron and sponge iron may be used as the source of iron. Electrolytic nickel may be the source of this element. Low-carbon ferromolybdenum may be the source of molybdenum and part of the iron. Titanium, if included, may be provided by high purity titanium metal or by lowcarbon ferrotitanium; aluminum, if included, may be provided by commercially pure aluminum; and vanadium, if included, may be provided by high purity vanadium metal or low-carbon ferrovanadium. It will be noted that the present alloy steel does not require cobalt and chromiurn, since it has been found that amounts as high as 6% of each of these elements do not affect the properties of the alloy.

The alloy steel is prepared by first melting the constituents, as in a high frequency induction furnace. While this may be done in air, it is preferably accomplished under an inert atmosphere, including a vacuum. Although, if the carbon content is too high it may be reduced by oxidation followed by deoxidation with, for example, carbon monoxide, aluminum or titanium, it is preferred to use starting materials sufficiently low in carbon such that oxidation and deoxidation are not required.

The molten material is then cast in a mold of, for example, sand, ceramic or metal, like cast iron.

The cast material is homogenized thermally at a temperature between about 1400 F. and about 1800" F. and then allowed to cool to room temperature to convert the substantially homogeneous austenitic face-centered cubic structure to a homogeneous body-centered cubic structure by means of a martensitic transformation. This solution annealing and martensitic transformation may be, and preferably is, preceded by a hot Working treatment at from about 1800 F. to about 2300 F. during which working one or more of the dimensions of the casting is reduced.

The martensitic alloy is then subjected to age-harden ing (maraging) by heating to between about 750 and about 1000 F. for from about 1 to about 10 hours.

The following examples are given for the purpose of illustration and are not intended to limit the scope of the invention in any Way.

Examples The alloys set forth in the following tables were prepared as follows: Electrolytic iron, electrolytic nickel, low-carbon ferromolybdenum (60% molybdenum and less than 0.02% carbon) and, where vanadium is included, low-carbon ferrovanadi-um vanadium and less than 0.06% carbon) are melted together under vacuum in a magnesia crucible held in a 9,600 cycle induction furnace. The furnace is then filled with argon at one atmosphere, and electrolytic manganese, high purity titanium, where titanium is included, and commercially For comparison, the average results from tests of nine examples of 18% nickel maraging steel made using the same procedure as in the foregoing examples and containing about 17-18% nickel, about 79% cobalt, about pure aluminum, where aluminum is included, are added 3-5 molybdenum and 0.12-0.19% titanium are as foland melted in. The melt is then cast in a cast iron mold lows: as a 15 lb. ingot having cross-sectional dimensions of UTS 201000 approximately 4" x 4". The ingot is then hot forged NTS 311,000 into bars having cross-sectional dimensions of approxi- NTS/{Yrs 155 mately 1 x 1", and rough machined in o rough te Elongation in 1 inch "percent" 14 bar blanks. These are then solution annealed for 1 hour Reduction in area do 62 at 1500 F., and allowed to air cool to room tempe au ture at the rate of about 10 F. per minute. The test bar The alloy steels of the present invention have a UTS blanks are then age-hardened at 900 F. for 3 hours and of at least about 160,000 p.s.i., preferably at least about allowed to cool to room temperature at the rate of about 190,000 p.s.i. and up to about 220,000 p.s.i.; an NTS 10 F. per minute. The blanks are then machined to the of at least about 250,000 p.s.i., preferably at least about final tolerances for testing, and, in the case of the bars 280,000 p.s.i., and up to about 310,000 p.s.i.; an NTS/ sed for m s ring n h ten il ng an annular UTS ratio of at least about 1.35, preferably at least about notch is cut. 1.5 and up to about 1.65; an elongation in 1" of at least Strength, in terms of ultimate tensile strength (UTS) about 11%, preferably at least about 13%, and up to is measured by ASTM Standard E8-57T, Tension Testabout 18%; and a reduction in area of at least about ing of Metallic Materials (A 370-54T, Mechanical Test- 30%, preferably at least about 50%, and up to about ing of Steel Products). Toughness is indicated by the 65%. ratio of notch tensile strength (NTS) to ultimate tensile The preferred composition of the present alloy steel strength, notch tensile strength being measured accordi about 7.5-9% ni kel; about 3.5-5% molybdenum; ing to Materials Research and Standards, March 196 about 2.54% manganese; carbon in an amount less than PP- using a j r diameter of 0.252", a minor 0.03%; less than 0.15% silicon, and the balance essendiameter of 0.177" and a notch radius of about 0.001". tially iron. Ductility is determined by the percent elongation in one Modification is possible in the selection of constituents inch on a 0.25" diameter test bar and the percent reducand amounts thereof as Well as in the particular techtion in area, both measured during the test for ultimate niques employed in preparing the product without departtensile strength referred to above. ing from the scope of the invention.

In all the following alloys of the invention, the amounts What is claimed is: of nickel, manganese, molybdenum, titanium, vanadium 1. An age-hardened, martensitic iron-base alloy conand aluminum are as set forth in the tables (in terms of sisting essentially of, in percent by weight, from 6 to percent by weight by chemical analysis), and the bal- 13% of nickel, from 2 to 6% of molybdenum, from 1.5 ance is essentially iron. Carbon is always less than 0.03% to 4% of manganese, no more than 0.5% of vandium, and silicon is always less than 0.15%. no more than 0.3% of aluminum, no more than 0.4% of TABLE A Example 1 2 3 4 5 6 7 8 9 10 Ni 6.3 6.2 8.3 8.1 8.1 8.2 9.9 10.0 12.4 8.1 4.1 4.1 4.1 4.1 4.0 4.1 4.0 4.1 4.0 4.0 2.7 3.9 1.8 3.3 3.6 2.8 1.8 2.5 1.5 2.9 0.19 0.20 0.19 0.19 0.19 0 0.19 0.18 0 0.48 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.02 0.02 0. 02 0.02 0.02 0.17 0. 02 0. 02 0.16 0. 02 4.8 6.0 4.6 6.0 6.3 5.5 5.1 6.1 5.6 5.7 UTS (Thousand .s.i.). 126 180 140 208 213 185 140 188 168 181 Percent elongation in 1 inch. 23 15 21 12 12 13. 5 20 13 15 14. 5 Percent reduction in area 76 61 73 43 32 53 74 59 64 61 It will be noted from the foregoing that Examples 1, titanium, carbon in an amount less than 0.03%, less 3 and 7, with low manganese equivalents, exhibit loW than 0.15% of silicon and the balance essentially iron, strength. the manganese content plus one-third the nickel con- Alloys prepared as the above example, but containing tent being from 5.5 to 6.2, and said alloy having an ultino aluminum, possess the same advantageous properties. mate tensile strength of at least about 160,000 p.s.i., a In the following Table B, the molybdenum content is 0 notch tensile strength of at least about 250,000 p.s.i., a varied in alloys in which the other constituents are esratio of notch tensile strength to ultimate tensile strength sentially constant. of at least about 1.35, an elongation in 1 inch of at least about 11%, and a reduction in area of at least about TABLE B 30% 65 2. An age-hardened, martensitic iron-base 3110 con- Example 11 12 13 14 15 sisting essentially of, in percent by weight, about 7i 5-9% N1 8 l 8 1 8 1 8 O 8 3 of nickel, about 3.55% of molybdenum, about 2.54% 11101::IIIIIIIIII""'" 'IIIII 2:1 3:1 414 515 6:0 of manganese, carbon in an amount 1688 than 0 243 28 less than 0.15% of silicon and the balance essentially V... 0.2 0.2 0.18 0.10 0.18 A1, (11 non, the manganese content plus one-third the nickel t l 'i t ili usand 121 12 12%; 3? 8% Content being from 5.5 to 6.2, and said alloy having an NTS (Thousand p.s.i.) 188 253 262 285 276 ultimate tensile strength of at least about 160,000 p.s.i., 1 31233? iilil clltiiill aiifij1:13:11:i 32 it 33 13 ii a notch tensile strength of at least about 0 p- NTS/UTS 55 55 55 a ratio of notch tensile strength to ultimate tensile strength of at least about 1.35, an elongation in 1 inch of at least about 11%, and a reduction in area of at least about 30%.

3. The alloy of claim 2 having an ultimate tensile strength of at least about 190,000 p.s.i., a notch tensile strength of at least about 280,000 p.s.i., a ratio of notch tensile strength to ultimate tensile strength of at least about 1.5, an elongation in 1 inch of at least about 13%, and a reduction in area of at least about 50%.

4. The method of making an iron-base alloy of high strength, toughness and ductility which comprises preparing a molten mass consisting essentially of, in percent by Weight, from 6 to 13% of nickel, from 2 to 6% of molybdenum, from 1.5 to 4% of manganese, no more than 0.5% of vanadium, no more than 0.3% of aluminum, no more than 0.4% of titanium, carbon in an amount less than 0.03%, less than 0.15% of silicon and the balance essentially iron, the manganese content plus one-third the nickel content being from 5.5 to 6.2;

pouring the molten mass into a mold and permitting it to solidify therein; thermally homogenizing the resulting cast structure; converting the homogeneous cast to a martensitic structure; and age-hardening the martensitic cast to provide an alloy having an ultimate tensile strength of at least about 160,000 p.s.i., a notch tensile rength of at least about 250,000 p.s.i., a ratio of notch tensile strength to ultimate tensile strength of at least about 1.35, an elongation in 1 inch of at least about 11%, and a reduction in area of at least about S. The method of claim 4 wherein the molten mass consists essentially of about 7.5-9% of nickel, about 3.55% of molybdenum, about 2.54% of manganese, carbon in an amount less than 0.03%, less than 0.15% of silicon, and the balance essentially iron.

References Cited UNITED STATES PATENTS 457,205 8/1891 Marbeau -123 2,516,125 7/1950 Kramer et a1. 75123 2,865,740 12/1958 Hegel et a1. 75-123 3,318,690 5/1967 Floreen et al. 75-123 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. AN AGE-HARDENED, MARTENSITIC IRON-BASE ALLOY CONTISITING ESSENTIALLY OF, IN PERCENT BY WEIGHT, FROM 6 TO 13% OF NICKEL, FROM 2 TO 6% OF MOLYBDENUM, FROM 1.5 TO 4% OF MANGANESE, NO MORE THAN 0.5% OF VANDIUM, NO MORE THAN 0.3% OF ALUMINUM, NO MORE THAN 0.4% OF TITANIUM, CARBON IN AN AMOUNT LESS THAN 0.03%, LESS THAN 0.15% OF SILICON AND THE BALANCE ESSENTIALLY IRON, THE MANGANESE CONTENT PLUS ONE-THIRD THE NICKEL CONTENT BEING FROM 5.5 TO 6.2, AND SAID ALLOY HAVING AN ULTIMATE TENSILE STRENGTH OF AT LEAST ABOUT 160,000 P.S.I., A NOTCH TENSILE STRENGTH OF AT LEAST ABOUT 250,000 P.S.I., A RATIO OF NOTCH TENSILE STRENGNTH TO ULTIMATE TENSILE STRENGTH OF AT LEAST ABOUT 1.35, AN ELONGATION IN 1 INCH OF AT LEAST ABOUT 11%, AND A REDUCTION IN AREA OF AT LEAST ABOUT 30%. 